How do you power a spacecraft in deep space? NASA's Voyager 1, which was launched three decades ago and is now about 10 billion miles (16 billion kilometers) from Earth, is using inertia from its blast off as well as the gravitational fields of the planets it has passed to speed toward the edge of the solar system at 10.6 miles (17.1 kilometers) per second. Unfortunately, it has taken Voyager 1 so long to get to its current location that by 2025 the space probe will have consumed all of the power provided by its radioisotope thermoelectric generators, or RTGs (which convert heat from decaying radioactive materials such as plutonium into electricity), and will no longer have enough energy to run its scientific instruments or send messages back to NASA as it crosses into interstellar space. Any missions hoping to go farther will need a means of acceleration that can get the spacecraft to the edge of explored space faster—before it has depleted its radioactive power source.

Many believe the answer lies in using a space sail that harnesses the sun's energy to keep a spacecraft accelerating long after it escapes Earth's atmosphere and reaches deep space. Heading beyond the sun's influence, the spacecraft would have attained a faster cruising speed than Voyager 1 and could maintain it via inertia with plenty of life left in its RTG and/or other generators to power its instruments and radio to communicate with Earth, allowing it to continue its mission for many decades to come. NASA is preparing to test a new space sail this summer that uses sunlight to provide this acceleration, whereas a scientist at the Finnish Meteorological Institute is refining a design for a different type of space sail that would have spacecraft surfing the solar wind, a continuous stream of charged particles that emanates from the sun, at speeds nearly twice that of Voyager 1 (18.6 miles, or 30 kilometers, per second).

The concept of space sails dates back the 16th century, when German astronomer Johannes Kepler first came up with the idea of using the sun's energy to propel objects through space. Today, there are three main types of space sail: solar (the one NASA plans to test), electric (being developed in Finland) and magnetic (a concept popular in the 1980s that never quite took off).

Scientists at NASA's Marshall Space Flight Center in Huntsville, Ala., and Ames Research Center at Moffett Field, Calif., within a week plan to test their solar sail, which will travel to space onboard a Falcon 1 rocket developed by Space Exploration Technologies (SpaceX) of Hawthorne, Calif. NASA says that during its two-week mission the 10-pound (4.5-kilogram) NanoSail-D solar sail will be the first ever be fully deployed in space and the first to use sunlight as a primary means of flight or orbital maneuvering. The kite-shaped sail is constructed of aluminum and plastic and unfurls to about 100 square-feet (9.3 square-meters) of light-catching surface. NASA has already spent more than $2.3* million developing its NanoSail-D technology.

The NanoSail-D may be followed over the next three years by another solar sail, this one built by The Planetary Society, a nonprofit space advocacy organization in Pasadena, Calif. The group is designing the electronics, solar array and solar sail motors for the Cosmos 2, which it hopes to launch with a Russian Soyuz–Fregat booster rocket, similar to the technology used to take astronauts, cosmonauts and tourists to the International Space Station. The Cosmos 1 solar sail was shot into space in 2005 (after five years of development and a failed launch in 2001) onboard a Russian Volna rocket. The submarine-launched Volna, however, malfunctioned and never made it into orbit, so the sail did not have the chance to deploy and demonstrate solar sail propulsion.

Louis Friedman, executive director of the Planetary Society, says that the sails must be launched to test them, because there is no solar wind in Earth's atmosphere. Russia and Japan have tested technologies that are similar to space sails, but because these tests were conducted near the Russian Mir space station (no longer in orbit) or in suborbital flight, "they aren't solar sails truly," he says.

Pekka Janhunen, an academy research fellow at the Finnish Meteorological Institute in Helsinki, meantime, is developing an electric space sail that uses the solar wind as its thrust source and does not need fuel or propellant. He says the sail will consist of up to 100 aluminum or copper alloy wires—each about 12.4 miles, or 20 kilometers, long and 20 microns in diameter (one micron equals about one four hundred-thousandths of an inch)—attached to reels that can adjust wire length. Each wire would be charged at up to 20 kilovolts via a solar-powered electron gun attached to the sail. The electrical charge emitted by the wires would react with the solar wind's plasma to generate momentum. Janhunen says it would take a 440-pound (200-kilogram) spacecraft using an electric sail less than five years to travel between Earth and dwarf planet Pluto.

It's too early to estimate the cost of building an electric sail, he says, but notes that it would be as much as four times cheaper to construct and fly a spacecraft attached to an electric space sail, because it would be lighter than conventional spacecraft and require only enough fuel to launch it out of Earth's atmosphere. Increasing thrust once in space could be a matter of adding more wires to the sail or making the wires longer.

Janhunen's work is moving from the drawing board to the lab and he hopes to build a prototype within three years that he could fly outside Earth's atmosphere to test the thrust produced by the electric sail. If his theory proves correct (and he can use long, electrified wires to take advantage of the solar wind), it would be a few more years before he and his colleagues could build the equipment required to mount the wires in the shape of a sail and test the structure's ability to propel some type of spacecraft.

A magnetic space sail would also rely on the solar wind for momentum. Robert Zubrin, founder and president of the Mars Society and president of Pioneer Astronautics, an aerospace R&D company located in Lakewood, Colo., and Dana Andrews, chief technology officer of Andrews Space, Inc., in Seattle, Wash., conceived the magnetic sail in the 1980s as a way to improve on the solar sail design. Instead of relying on sunlight, it would use the solar wind's plasma (which travels at 310 miles, or 500 kilometers, per second) as its means of propulsion.

Taking advantage of the fact that plasma is deflected by magnetic fields, Zubrin and Andrews proposed putting a magnetic field around a spacecraft like a bubble and having the solar wind push the spacecraft. There was, however, one small hitch: the superconducting wires required create a magnetic field have never been developed. (The project briefly received some funding from the NASA Institute for Advanced Concepts, but that organization was shuttered last year.) "The magsail is a concept waiting for a technology," Zubrin says.

The European Space Agency studied the magnetic sail concept to use during a manned trip to Mars but ultimately came to the same conclusion as Zubrin.

Of course, none of these sails are effective once they are out of the sun's range. By the time any spacecraft reaches about 37 billion miles (60 billion kilometers) from the sun "the solar wind becomes very tenuous and then stops," Janhunen says. At that point, he envisions his electric space sail being jettisoned from its spacecraft so that its momentum would fly it beyond the solar system, tapping an onboard source of nuclear energy (similar to Voyager 1) to power its systems.

The reward for building a successful space sail would be to increase the velocity of a spacecraft indefinitely without the need for propellant. "This would let you go enormous distances at enormous speeds," the Planetary Society's Friedman says. "It's the only technology that we know of that can one day take us to the stars."

*Correction (8/01/08): This article originally identified the cost as $30 million.